The present invention relates to a single crystal pulling apparatus for growing and pulling up a single crystal ingot from melt of a polycrystal substance based on the Czochralski (CZ) technique. In particular the invention relates to such single crystal pulling apparatus wherein granular polycrystal substance is continuously added to the melt for replenishment.
Such a single crystal pulling apparatus comprises a heating chamber wherein a single crystal is grown from molten polycrystal substance (a raw material such as silicon). In the heating chamber are provided elements such as a quartz crucible, a cylindrical heater surrounding the crucible, and a cylindrical thermal insulator (heat shield) surrounding the heater. Beneath the heating chamber are provided mechanisms for rotating the crucible together with a vertical crucible shaft, on which the crucible is fixedly mounted, to control the uniformity of the heat flux in a polycrystal molten liquid (melt) during the crystal growing operation.
The polycrystal substance charged in the quartz crucible is heated and melted down by the heater to turn into a polycrystal melt, and in this liquid is dipped a seed crystal fixed at the lower end of a pull means such as a wire, and the desired single crystal grows from the bottom tip of the seed crystal as the pull means is rotated and drawn up at predetermined rates together with the seed crystal.
In the CZ single crystal growth technology as described above, the segregation coefficients of dopants do not exceed 1 (e.g., in the case of phosphate 0.35 and in the case of boron 0.75), so that the dopant concentration of the melt soars as the single crystal growth proceeds. As a result, the resistivity distribution in the ingot tends to be such that the closer to the bottom of the ingot, the lower is the resistivity. Thus, in cases where it is required that the resistivity values throughout the cylindrical portion of the ingot must be within a certain range, the length of the cylindrical portion worth growing is limited.
Also, in the CZ single crystal growth technology, a single crystal ingot is grown in a manner such that firstly a conical portion is formed by gradually increasing the diameter of the growing crystal to a predetermined size after narrowing it in the range of 2 to 3 mm in diameter so as to prevent propagation of dislocation into the growing crystal, secondly a cylindrical portion is formed, and finally a tail portion is formed by gradually decreasing the diameter, since if the growth operation is terminated and the single crystal is removed from the melt immediately after the desired cylindrical portion is grown, the thermal stress thereby caused is of such magnitude that slip occurs in the single crystal body. Naturally the shorter the cylindrical portion, the fewer the wafers that can be obtained from the single crystal ingot.
In order to constrict the resistivity within the acceptable range so as to maximize the worth-growing length of the grown ingot and thus the number of wafers obtained from the ingot, a duplex crucible system has been adopted; according to which a cylindrical internal crucible is concentrically provided in the quartz crucible to form a duplex structure. In this system, during the growth of the single crystal, an appropriate amount of granular polycrystal substance with or without dopant is poured outside the internal crucible in a continuous manner, and thus the resistivity and concentration of interstitial oxygen become uniform along the growth axis of the ingot.